Field of the Disclosure
The present disclosure relates to a toner used for electrophotographic image forming methods that can be applied to copy machines, printers, facsimiles, and multifunctional machines used in these apparatuses.
Description of the Related Art
As well as being used for copying a document, electrophotographic image forming apparatuses including copy machines are used as information output apparatuses connected to other information apparatuses by digitizing the information. Accordingly, the toner used in such electrophotographic image forming apparatuses is increasingly required to exhibit high performance in forming high-quality images with high definition at a high speed with high reliability.
In particular, since electrophotographic apparatuses are being increasingly used in a variety of environment as the market of the apparatuses is growing, the apparatuses are required to form images having stable quality independent of the environment.
In addition, in view of reliability, it is required to provide the apparatuses that can form images without degrading image quality over a long time.
For example, it has been known that toners used for electrophotographic applications deteriorate in chargeability in high-humidity environment. This deterioration can negatively affect the resulting image quality. For example, fogging can occur. Fogging is a type of scumming caused by a portion of the toner slightly developed in a blank region that is not intended to be printed.
Typically, an external additive made up of functional particles is applied to the surfaces of the toner base particles (resin particles) to impart fluidity and chargeability and to function as a spacer between the toner and members of the image forming apparatus. The toner however receives a shear stress in an image forming apparatus while being used for outputting images over a long time. Consequently, the external additive is removed from the base particles or in the base particles. This is the cause of reduced chargeability and fluidity and deterioration of the function as the spacer between the toner and the photosensitive drum, thus making it difficult to transfer all the toner on the photosensitive drum to a recording medium or an intermediate transfer member, that is, reducing transfer efficiency. Consequently, the quality of the resulting image can be degraded. For example, the evenness in image density is extremely degraded in a high-density region of the image.
The fixing system of image forming apparatuses is being changed from a conventional system using a hard roller having a high heat capacity to a light-pressure fixing system using a fixing film or belt having a low heat capacity, from the viewpoint of energy saving for reducing power consumption.
In the light-pressure fixing system, the heat capacity of the fixing member is reduced from the viewpoint of reducing the time taken to raise the temperature of the system to a fixing temperature set (controlled) for fixing, and of enabling quick start. If the heat capacity of the fixing member is reduced, the temperature of the fixing member decreases more than in a conventional hard roller system when continuous high-speed copy is made. Accordingly, a toner that can be fixed at a lower temperature is required, and it is desirable that the low-temperature fixability of the toner is further increased.
From the viewpoint of reducing the heat capacity, a durable toner having good environmental stability of chargeability and capable of outputting images over a long time without much degrading the image quality and having good low-temperature fixability is more desired than ever, and a variety of attempts have been made.
Japanese Patent Laid-Open No. 2004-219507 discloses a toner using a cycloolefin copolymer as a binder resin. Cycloolefin copolymer is colorless and transparent, or has a high light transmittance, and is less hygroscopic. According to the description of the above-cited patent document, by using such a cycloolefin copolymer as a binder resin in a toner and further adding a polypropylene wax or a polyethylene wax as a releasing agent and an organic boron compound as a charge control agent, a toner superior in productivity, storage stability, fixability, transparency, and environmental stability can be provided. In addition, a developing device in which the occurrence of fogging and ghosts is prevented can also be provided.
Even in this toner, further improvement is desired for reducing the deterioration in transferability that can occur when images have been output over a long time. The deterioration in transferability can be caused by a phenomenon in which the external additive is buried in the toner base particles.